In recent years interest has grown in the use of dry cooling towers for steam power plant cooling. The term "dry cooling towers" refers to cooling towers that use only air as the cooling fluid. The principal reason for this growth in interest in dry cooling towers is the projected increase in competition for existing water supplies from rivers and lakes between the population, industry and agriculture. In fact, in certain locations in the United States where water supplies are very scarce, all power plants must have dry cooling towers by law.
One of the problems associated with using dry cooling towers, as opposed to an equivalent wet (water) cooling tower, is the severe economic penalty resulting from the higher heat rejection temperature in warm weather. Higher heat rejection temperatures reduce plant output when power demands are likely to be the highest. This loss of capacity would have to be compensated for by excess capacity in the plant or by the purchase of power from other utility systems.
Because of these economic penalties, there is a strong incentive to augment these dry cooling towers through evaporative cooling on hot days using inexpensive water made available at the plant site. These combined wet-dry cooling systems increase plant efficiency and power loss problems can be ameliorated by using only a relatively small quantity of available water when compared to the continuous usage in all-wet cooling towers.
Several methods have been suggested to combine wet and dry cooling systems to satisfy this need. These include: (1) separate wet and dry cooling towers, (2) integrated wet and dry cooling towers, (3) dry cooling towers used with water cooling ponds, (4) dry cooling towers using a water spray over the dry cooling heat exchangers, (5) dry cooling tower systems having a water deluge over the heat exchangers, and (6) water augmented dry towers with active heat transport systems. Many factors must be considered in selecting the most economic advantageous option for a given plant site. These factors include water availability, fuel cost, water cost, elevation and climatology.
One type of water augmented cooling tower system is referred to as a separate channel augmented tower (SCAT) developed in part by the Department of Energy. In this system the basic heat exchanger for the cooling tower is a multi-ported aluminum extruded tube that has integral fins chipped from the tubes. The extruded tube has a plurality of side by side channels. Most of the channels convey ammonia and selected ones of the channels are water channels to increase the cooling of the ammonia by heat transfer to the water. The water is piped to a wet cooling tower either inside the dry tower or to a secondary wet tower outside the dry tower. In this system the air path through the heat exchanger must be very long due to the fact that the air must pass not only across the ammonia channels but also across the water channels. When the water channels are not in use and become passive, there is no heat transfer between the ammonia and the air through the water channels.
It is a primary object, therefore, of the present invention to ameliorate the problems in prior combined wet-dry cooling towers for condensing the exhaust steam in power plants.